1. Field of the Invention
The invention relates to sliding bearing structure for a shaft member, and more particularly, to a sliding bearing structure for a shaft member, that is used in a vehicular internal combustion engine or the like.
2. Description of Related Art
With a typical sliding bearing structure for a shaft member, lubricating oil is supplied to form a lubricant film in a clearance between a sliding surface of a shaft member and a sliding surface of a bearing member. Supporting a load by the oil film pressure that is created in this lubricant film reduces friction loss and helps to prevent wear and seizure.
In order to keep lubricating oil in the clearance between such sliding surfaces, a bearing structure in which many narrow grooves or striations are formed in parallel in the circumferential direction in the inner peripheral surface of the bearing has been proposed.
Also, Japanese Patent Application Publication No. 2007-285456 (JP 2007-285456 A) describes a bearing structure that obtains a lubrication characteristic, reduces surface pressure, and improves seizure resistance, by forming a concave portion in a bearing member that contacts a rotating shaft and putting a contraction member that is made of material with a larger coefficient of thermal expansion than the bearing member in the concave portion, such that the concave portion becomes an oil reservoir when the bearing portion is cold, and forms a flush sliding surface by the expansion of the contraction member when the bearing portion heats up.
In a vehicular internal combustion engine or the like, after the engine has warmed up, there is not that much sliding resistance (i.e., friction loss) at a sliding bearing, but during a cold start from an extremely low temperature (approximately −30° C.) to room temperature (approximately 20° C. to 25° C.), for example, there is an extremely large amount of sliding resistance. This is because the viscosity of the lubricating oil depends on the temperature, and when the temperature is low, the viscosity drastically increases.
Therefore, in order to reduce the sliding resistance at such low temperatures, it is preferable to raise the temperature of the bearing portion early on. However, when the temperature is low, the temperature of the supplied lubricating oil itself is also low, so an increase in temperature is delayed, and even if the temperature of the lubricating oil is increased by heat generated from shearing resistance, this lubricating oil ends up quickly flowing out of the bearing portion, so it takes time to raise the temperature of the bearing portion.
On the other hand, at a high temperature (approximately 80° C. to 120° C.) during high speed operation or steady operation after the engine has warmed up, the temperature may rise excessively, resulting in seizure or the like, unless there is a sufficient amount of lubricating oil, so a sliding bearing structure that displays sufficient cooling performance at high temperatures is needed.
The bearing structure described in JP 2007-285456 A has a contraction member that is made of material with a larger coefficient of thermal expansion than the bearing member set in a concave portion that is formed in a bearing member that contacts a rotating shaft, and the concave portion becomes an oil reservoir when the bearing portion is cold. The bearing structure is not intended to raise the temperature of the bearing portion early on.